The mammalian inner ear is a dual function organ comprised of the vestibular apparatus that senses spatial orientation and the cochlear duct which functions in sound transduction. The development of the cochlea is of particular interest due to the high incidence of hearing loss that occurs in the general population. The cochlear duct arises from a ventral outgrowth of the inner ear anlage, the otic vesicle. During its development the cochlea extends into a coiled tube that will contain the sensory hair and support cells of the Organ of Corti that are the transducers of sound. The function of the Organ of Corti is tightly coupled to the proper timing of events that occur during its formation. Sensory cell precursors must be specified from the otic epithelium, undergo cell-cycle exit and then differentiate into hair and support cells. The precise timing of these events is crucial for the proper development of the Organ of Corti, the cochlea as a whole, and, subsequently, the sense of hearing. While many of the genes responsible for each of these developmental events have been discovered the factors controlling their timing have yet to be identified. Recently, our collaborator, Dr. Ian Krantz (Children's Hospital of Philadelphia) identified mutations in the Epithelial Splicing Regulatory Protein 1 gene in several individuals with profound sensorineural deafness. In an effort to functionally link the deafness phenotype with mutations in Esrp1 we have begun to analyze mouse mutants in Esrp1 and/or Esrp2 for inner ear phenotypes. My preliminary results indicate that Esrp1 mutants exhibit a truncated cochlear duct and a significant delay in the development of auditory hair and support cells. The goal of this proposal is to determine the mechanism by which Esrp1 regulates the timing of sensory development in the inner ear. To accomplish this objective, I will systematically analyze the inner ears of Esrp mutants for alterations in the specification and cell-cycle exit of sensory progenitors, as well as the differentiation and maturation of cochlear hair and support cells (Aim1a). I will also perform RNAseq experiments on cochlear ducts from Esrp1-/- and control embryos to identify aberrantly spliced mRNA transcripts and potential targets of Esrp1 (Aim 1b). Finally, given the neonatal lethality of Esrp1-/- mutants from a cleft palate defect, I will generate a conditional knockout of Esrp1 in the inner ear to interrogate its requirement for hearing in adult mice (Aim 2). These experiments will uncover unique roles of Esrp genes in inner ear development and highlight the mechanism by which pathogenic mutations in Esrp1 result in deafness.